Many systems require a backup battery to maintain system operation in the event of the loss of primary power. For example, in radio telephone equipment, where the link (also called a modular cluster) between telephone subscribers and the central switch is a wireless local loop, it is desired that telephone service be maintained in the event of primary power failure. A radio telephone system of a type for use with a backup battery is illustrated in U.S. Pat. No. 4,675,863. If primary AC power fails, the backup battery is used to maintain uninterrupted telephone service. To be useful, the backup battery must be maintained in a fully charged and operational condition to be ready in the event of primary AC power failure.
In the prior art, backup battery readiness is maintained by a continuous battery charger and a battery heater. The battery charger senses battery charge, and maintains a full battery charge without overcharging the battery. Following a primary AC power failure, when AC power is restored, the battery charger restores the battery to full charge. Similarly, if charge leaks off the battery with time, the battery charger periodically restores the battery to full charge.
Battery efficiency falls off dramatically at low temperatures. That is, if the backup battery is too cold, it will not operate the equipment for very long in the event of primary AC power failure, even if the battery is fully charged. Therefore, in addition to maintaining full battery charge, it is necessary to heat the battery to a high enough temperature to maintain backup battery power readiness.
In the prior art, the battery heater is typically a resistive heating element in close proximity to the battery. The heating element is controlled in a closed loop temperature control system. If the sensed battery temperature falls below some minimum lower level (e.g. 0 degrees centigrade), the battery heating element is enabled. When the sensed battery temperature rises above that level, the battery heating element is disabled. Thus, in cold weather, the battery temperature is maintained to a minimum of 0 degrees centigrade.